The invention relates to display and recording devices, particularly those in which images or patterns are formed by means of the electrophoretic migration of particles.
The use of the electrophoresis phenomenon in display devices containing an colloidal suspension layer between electrodes has been contemplated in the prior art, such as represented by U.S. Pat. Nos. 3,612,758 and 3,668,106. These patents are hereby incorporated by reference to provide background information on the chemistry and technology of electrophoretic displays.
A simple electrophoretic image display (EPID) device is shown, which is a passive, light scattering-type display (FIG. 1). This EPID device is implemented as a cell which consists of a colloidal suspension of pigment particles in an organic liquid held between two electrodes, at least one of which is transparent. In one embodiment, the pigment particles are negatively charged with respect to the liquid. If a positive potential is applied to an electrode in electrical contact with the suspension, the negatively charged pigment particles will be attracted to that electrode. A negative potential on an electrode repels the particles.
One side panel of the device is made of a transparent material such as glass, coated with a transparent, electrically-conducting layer. When light-colored pigment is attracted to this side panel electrode, the observer looking at the panel sees the reflected color of the pigment. A dark-colored dye is included in the suspending medium, so that the pigment at the back of the cell is obscured by the opaque dye and the observer sees only the reflected color of the dye. When the polarity of the voltage on the electrodes is reversed, the position of the color is reversed. The described EPID display cell also possesses "memory" since the particles remain on the electrodes after the applied voltages are removed due to chemical, electrical, or van der Waals forces.
U.S. patent application Ser. No. 854,117, filed on Nov. 23, 1977 by Dalisa and Singer and assigned to the assignee of the present application, (which application is hereby incorporated herein by reference and is included in the present specification as if set forth at length herein) discloses an electrophoretic image display ("EPID") device which contains an opaque insulating fluid containing particles of at least one color pigment in suspension, essentially all such particles having a charge of one polarity. The device further includes a main first electrode, main second electrodes spaced from the first electrode, with the fluid disposed between the first and second electrodes. An insulating layer is adherently provided at least partially over the second electrode.
A control third electrode is interposed between the first and second electrodes, and is separated from the second electrodes by the insulating layer, for selectively adjusting the electric field between the first and second electrodes to establish a threshold for transporting the particles, causing an image to be formed on the main electrodes by the presence or absence of the particles.
Another important feature of the invention of the above application is an electrophoretic display device having a first plurality of electrodes including spaced conductive elements insulated from each other. There is provided a dielectric fluid having pigment particles homogeneously dispersed therein with a charge furnished to individual ones of the particles. The device includes a second electrode having conductors spaced from the first plurality of electrodes by the dielectric fluid. An electrical field is applied across selected ones of the first electrodes and second electrode whereby charged particles migrate along the electric field electrophoretically in imagewise configuration to one of electrodes.
The electric field between the first plurality and the second electrodes is adjusted for controlling the migrating particles.
While the device disclosed in the above application of Dalisa et al are highly desirable and provide a significant advance in the art, they, as well as other EPID devices that operate by scattering light, are not always desirable because they require ambient illumination and cannot be used in a back-lighted operation. The presently disclosed light-transmitting EPID devices overcome this shortcoming and provide additional benefits by not requiring the presence of an opaque dye in the insulating fluid so as to prevent light reflected from colloidal particles that are not present at "written" areas of the display, from reaching the viewer.